Electric coupling circuits



June 13, 1933.

1 H. A. WHEELER ELECTRIC COUPLING CIRCUITS Fil ed Jan. 16, 1931 2 Sheets-Sheet 1 INVENTOR HAROLDAJVHEELER BY QMJM- M +M.

ATTORNEYS June 13, 1933-. H. A. WHEELER I ELECTRIC COUPLING CIRCUITS Filed Jan. 16, 1931 2 Sheets-Sheet 2 INVENTOR HARULD A. WHLER BY p g MJ-M ATTORNEYS Patented juneylii, 1933 I r UNITED sTATEs PATENT OFFICE nan/L1: a. WHEELER, or Guitar imcx, NEW ou, nssmuon 10 HAZELTINE CORPORATION, a CORPORATION or DELAWARE morale COUPLING cmcurrs a lication flied January 16,. 1931. semi Io. 500L110.

This invention relates to high frequency 'electrie coupling circuits tunable over a range in frequency and adapted more especially for interconnecting successive elements of a high frequency thermionic amplifier or a radio system.

The couplingcircuits, of the present invention are characterized in that the voltage amplification produced thereby is caused to vary automatically with the frequency of tuning in a manner which is under the control of the designer; thedesign in general a being preferably such as to produce a substantially. constant resultant amplification, i.ej. uniform sensitivity throughout the tun? able frequency range. Y

The subject matter of the present invention has been published in a paper entitled Theory and operation of tuned radio frequency couplings stems b 'H.'A. Wheeler and W; A. Mac onald. presented and distributed in printed form at the 1930 convention ofthe Institute of Radio Engineers held at Toronto Canada.

With an elementary type ofv high frequency couplin winding [trans ormer havin a tunable secondary is a typical examp e, the varia-' tion in amplification@with frequency is not under the control'of the designer, but.increases with frequency throughout the tun- ,able range in a manner well understood,

I cuit referred to above t with consequent detrimental; effects on the amplification, selectivity and stability of the system.

In the elementary type of coupling cirere, exists between the primary and tunable secondary circuits a coeilicient of couplin 'which is substantially constant throug out the frequency range, a condition which is partially responsible for the mentioned increase in amplificationwith frequency. The present invention avoids the above noted objections by proper cooperation of different reactance elements in the primary circuit. These provide an effective coupling between input andtunable output circuits which automatically decreases with increase in the tuned frequency, and maybe proportioned to ofl'set his paper was circuit of which a twonecessity increases with frequenc ubtracted from the substantially invariable v14, 1926 and Serial No. 120,045 filed July 2,

The Trube circuits are characterized in the provision of a tunable secondary circuit hav nga primary circuit coupled thereto at -least"a portion of which is resonant below the tunable range for roviding a coupling component between primary and secondary circuits which decreases with increase in the tuned frequency. This variable coupling component supplementedb a fixed coupling provides a resultant coup ing which automatically falls off at a pro-selected rate as the tuned frequency is increased.

. With the circuits of the present invention on the other hand, a tunable secondary circuit is coupled to a lprimary circuit at lea'sta portion of whic is resonant above the tunable frequency range for provid-' ing a coupling component between primary and secondary circuits which'increases with the tuned frequency. This coupling component increasing with frequency issijitracted from a fixed component to provide a resultant coupling-which, as in the case of' the Trube circuits, autbmatically falls ofi at a preselected rate with increase in the tuned frequency.- i

In certain embodiments of the presentinvention the primary circuit is coupled both inductively and capacitively tothe tunable secondary circuit, witli the capacitive coupling arranged to oppose the magnetic coupling. The capacitive coupling which of is thus magnetic coupling to provide the mentioned verse order of the tuned frequency;

resultant coupling which varies in the in- 1 With certain other embodiments of the present invention the magnetic coupling-between primary and secondary circuits is absent; in fact there are no clearly defined primar and secondary circuits, but rather a tuna le network containing reactive elements interposed between in ut and output terminals. For circuits of t is type, the resultant coupling between input and output terminals is determined by the ratio of two reactive elements the impedances of which vary with fre'quency at such different rates as to provide the desired automatic variation in coupling between input and output terminals.

Referring now to the drawings:

Figs. 1, 2 and 3 show elementary coupling circuits in accordance with the present in vention wherein a primar circuit is coupled in opposed capacitlve an inductive relation to a tunable secondary circuit;

1 Figs. 4 and 5 show modifications of the invention wherein the ma etic coupling is absent, the coupling circuits being more in the nature of tunable networks interposed between input and output terminals;

Fig. 6 shows the circuit of Fig. 3 in association with an antenna system connected to the input terminals thereof; while Figs. 7 and 8disclose the mannerv of connecting the anode circuit of a thermionic element to the input terminals of the circuits of Figs. 3 and 4 respectively.

The-circuit of Fi 1 includes a secondar coil L and a varia 1e tuning condenser bridged between output terminals 0, the lower output terminal being conductively joined to the lower of a pair of input terminals I in order to rovide a common or ground connection.

"the secondary coil L is aprimary'coil L so connected between input terminals I that the upper input and output terminals are of like alternatin current polarity. A coupling condenser 5 of small ca acity is connected between the ungrounde terminals of coils L and L for providing a capacitive couplin between primary and secondary cir,

cuits w 'ich opposes throughout the tunable frequency range the magnetic coupling therebetween due to the mutual inductance M. An additional capacity C may be connected between the input terminals 1, or, as is more generally the case, a capacity 0 may exist between the input terminals in the operation of the coupling circuit due to the type of input circuit connected thereto.

In order, with the circuit connections specified, to assure that the capacitive coupling between primary and secondar circuits due to the condenser C will su stantially oppose the magnetic coupling therebetween due to the mutual inductance M, throughout the tunable frequency range, it is necessary that the inductance of the primary described in some detail as ductively related to coil L together with the associated capacities, including that of the coupling condenser C and any capacity C effectively bridged between theinput terminals, be resonant at a frequency above the tunable range as determined by adjustment of condenser C.

.resonance frequency of the primary circuit as determined by elements L (Land 0,, should be only slightly higher thanthe highest frequency within the tunable range. It will be apparent from the above considerations that whereas the magnetic coupling betweep primary and secondary circuits is substantially independent of the tuning adjustment, the opposed capacitive couplln of necessity increases with the tuned requency as a result of the inverse variation with frequency in the impedence of the coupling condenser C The combination of these two factors produces a resultant or effective couplin between input and output terminals which automatically decreases with increase in the tuned-freuency at a rate which is de endent upon t e relative proportioning o the reactive elements. q I

The coupling circuit of Fi 1 has been being generic to the t pe of coupling circuit disclosed herein. Ihe specific circuit of Fig. 1 however, constitutes no part of the present invention, having been heretofore broadly disclosedand claimed in a copending appqlication of Wm. A. MacDonald Serial 0. 469,040, filed July 19, 1930, now U. S. Patent 1,888,278, issued November 22, 1932; specific modifications thereof being disclosed and claimed in an application of H. A. Wheeler Serial No. 480,947 filed Sept.

The modification of Fig. 2 differs from that of Fig. 1 by the addition of the inductance L connected in series with the primer coil L between input terminals I. Coi L is uncoupled magnetically with respect to coils L and L as is indicated by drawing the coil with its axis at rightan les to the axes of coils L and L rimary coil L is resonant with its associated :capacities at a frequency considerably hi her than the tuning range. Coil L is a ded to reduce the primary resonant frequency to a value nearer to but still above the 'tuning range. In this way the effective opposing capacitive coupling can be controlled by the value of L without vary.- ing the inductive couplinfg M.

n the modification 0 Fig. 3 the condenser C instead of being connected to the upper terminal of the secondary coil L as in Fig. 1, is tapped to an intermediate point thereof. This change is in the direction 013 increasing the flexibility of the system since it provides a means for varying the capacitive cou ling between rimary and secondarycircuitswithout c anging the value of C or appreciably afiecting the resonance fre uency of the primary circuit.

'\%ith the circuits of Fi 1 to 3 inclusive, the total inductance in t e primary circuit should be of the same order ofmagnitude as the inductance of the secondary coil L.

. A moderate degree of coupling, ofthe order of 30 percent, should be emp oyed between primary and secondary circults. If the coupling is too close the circuits will be tuned as a unit thus preventingthe resonance freuency of the primary circuit from havin t e desired effect in controlling the ampl fication characteristics of the circuit. A de- Fig. 4 the mag 'L v between input terminals I. Condenser C, and primary coil L should be resonant at the mentioned freguency above the tunable ran e determine byadjustment of C.

"It wil be noted that m the modification of Fig. 4 two paths are.provided in parallel between input terminals I. One of these pafihs contains element L, and .C,, while the 'ot er (path contains the coupli condenser C, an the tuned circuit LC. no to the presence of condensers C and C, the impedances of both pathsfare caused to decrease with increase in the tuned-fr uency,

I but at such difl'erent rates that as t e frequency of tuning increases, an increasing percentage of the total current flowing past' the in ut terminals I will be shunted throng the rimary circuit path containing elements C leaving a decreasing ortion flowing through the tuned circuit C. Thus the efl'ective coupling between primary and secondary circuits automatically falls off as the frequency of tuning increases.

The circuit'of 'Fi 5 0 rates on somewhat the same princ ple. n'this modification L is tuned'by C, to a frequency above the tunable range; likewise L, is tuned by C to a frequency above the range. The resonance frequency of LC, is, however, further removed from the tuning range than that of L C so that as the circuit is tuned to higher frequencies by adjustment of C, the impedance of L 0 bridged between the input terminals will increase at a rate less between primar and secshunted through the impedance L 0 In this-we the effective coupling between in put an output terminals may be caused to decreasein a preselected manner with increase in the tuned frequency. v 7

Fig. 6 merely shows the modification of 'Fig.-3.with an antenna circuit Aconnected v to the input terminals thereof. The anten v na-to-ground capacit of the antenna circuit constitutes the capacity C,. The connection between the lower input and output terminals is not shown since a conductive 'ath through the ground connections is provided therebetween.

In Fig. 7 the anode circuit of a thermiv V is connected to the input teronic tube minals of the Fi 3 circuit. The upper indput terminal is Joined directly to the ano e, while'the.lower input terminal is connected to the -+B lead extending to a space current supply source for-supplying energy to'the space current path, including the primary coil L, and the grid-biasing resistor R joined to the cathode. Condensers D serve to by-pass the hi h frequency current. The direct connection tween the lower input and out ut terminals is omitted to ,pre-

vent groun ing the +B lead at the rotor element of condenser C. Condenser C servesas a blocking as well as a coupling condenser. InFig. 7 the ca acity C, of the coupling circuitis provide by the anodeto-lground tube'capacity.

Fig. 8 the anode circuit of a thermi-- onictube V is connected to the input terminals of the circuit of Fig. 4. In this instance it is necessary to em loy a choke coil L in order ,to complete e space current supply path owing to the presence of C in series with L, in the coup ing circuit.

The coupling circuits of the present in- 'vention are such that the rotor element of successfully when connected to 'a thermionic tube in the manner shown in Fig. 7; where-.

as a choke coil arrangement is required for the circuits of Fi s. land 5.

It is frequent y convenient and advantageous to employ inherent capacity in place of a physical condenser for C or in these circuits. This is particularly recommended in the case of C in Fi 1 to 8 inclusive and in the case of C in igs. 1, 2, 3, 5 and 7.

What is claimed is:'

1. A high frequency electric coupling circuit tunable over a range in frequency comprising a. variable tuning condenser connected between an output and a. common terminal, an inductance connected between an input and said common terminal, and a fixed'capacit'y shunted by a second inductance connected between said in ut and output terminals, said second. in uctance tol gether with said fixed capacity bein res 'onant at a frequency slightly above sai tunmg ran e.

2. A igh frequency electric coupling circuit tunable over a range in frequency comf prising a variable tuning condenser connected between an output and a common terminal, a first inductance shunted by a fixed capacity connected between an input and said common terminal, and a second in 15 ductance shunted by a fixed capacity connected between said input and output terminals, said second inductance together with its associated fixed capacity being resonant at a frequency slightly above said tuning 0 range, and said first inductance together with its associated capacity being resonant at a still higher frequenc In testimony whereof afiix m signature.

HAROLD A. W EELER. 

